A tridentate Ni pincer for aqueous electrocatalytic hydrogen production

Article abstract of DOI:10.1039/c2nj20912h

A Ni^II complex with a redox-active pincer ligand reduces protons at a low overpotential in aqueous acidic conditions. A combined experimental and computational study provides mechanistic insights into a putative catalytic cycle. The Royal Socie

Full text of DOI:10.1039/c2nj20912h

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LETTER
A tridentate Ni pincer for aqueous electrocatalytic hydrogen productionw
a
a
a
b
Oana R. Luca, Steven J. Konezny, James D. Blakemore, Dominic M. Colosi,
a
a
a
a
Shubhro Saha, Gary W. Brudvig, Victor S. Batista* and Robert H. Crabtree*
Received (in Gainesville, FL, USA) 25th October 2011, Accepted 29th February 2012
DOI: 10.1039/c2nj20912h
II
A Ni complex with a redox-active pincer ligand reduces protons at
a low overpotential in aqueous acidic conditions. A combined
experimental and computational study provides mechanistic insights
into a putative catalytic cycle.
we observe a first reduction wave just above 0 V vs. NHE for
complex 1 (Fig. 1) that we tentatively assign a ligand-centered
process given the literature precedent. The second reduction at
II
I
B ꢀ0.5 V vs. NHE is therefore assigned as a Ni /Ni couple.
To the precatalyst in acetonitrile increasing amounts of acid
were added and an increase in current response was observed by
cyclic voltammetry (ESI: S1-2w). Assuming a rate-limiting
chemical step, we applied the voltammetric kinetic treatment of
1
,2
2
H is currently produced by steam reforming of fossil fuels
which is both expensive and detrimental to the environment. If
H is to be a fuel in environmentally friendly alternative energy
2
1
5a,b
15c
3
,4
DuBois et al.,
that leads to kinetic parameters for the H
the DuBois case, our rate law follows eqn (1) and an apparent rate
originally developed by Nicholson et al., (S7w)
strategies,
more sustainable sources of H2 are required.
2
evolution reaction. Like
Elemental Pt is currently the best catalyst for the reduction
5
of protons to H
2
but its low abundance and high cost make it
4
+
3
unsuitable for global use. A range of different transition metal
constant of 1.05 (ꢁ0.21) ꢂ 10 has been determined for 0.1 M H ,
5
mM 1 at ꢀ250 mV vs. NHE. This corresponds to a voltammetric
ꢀ1 16
complexes can act either as electrocatalysts or photocatalysts,
9–11
In a recent
6
including systems involving Co, Mo and Ni.
7,8
rate of hydrogen formation of 105 s .
report, a tetradentate Co system with a redox active ligand
+
rate = k[H ] [cat]
2
1
2
(1)
also operates in aqueous conditions. Improved systems that
can operate in aqueous conditions with abundant first row
transition metals are of current interest.
Further information was obtained at variable potential where we
find sustained H evolution in a series of chronoamperometry
Pincer ligands are attractive because they are easy to
assemble from readily available materials and impart high
stability to the resulting complexes. Furthermore, their modular
2
experiments. A plot of current density vs. overpotential (Fig. S3-1w)
shows that the overpotential for 1 is a very satisfactory
1
3
ꢀ2
nature facilitates tuning of ligand properties. Here we report
that a Ni pincer gives good activity as an operationally homo-
geneous electrocatalyst for proton reduction in aqueous conditions.
Electrochemistry in acetonitrile was used to pinpoint redox
events of the metal complex in the presence of added acid and
subsequently aqueous conditions. Density functional calculations
140 mV at a current density of 1 mA cm , assuming a
thermodynamic potential of ꢀ84 mV vs. NHE under our
conditions (Details in S3w).
The compound can also operate in water. A high surface
area reticulated vitreous carbon working electrode was used to
2
determine quantitative H evolution from 1 via mass spectro-
(DFT) calculations offer insight into a possible mechanism.
scopy (see ESIw). Specifically, a 50 mL 0.1 M KCl/HCl
solution (pH 1) containing 0.2 mM precatalyst was held at
ꢀ
1.1 V vs. NHE for one hour. The charge passed in the
Results and discussion
catalytic experiment was 212 C. After subtraction of the
relevant 80 C background current, 132 C were consumed by
the catalytic chemistry. This is equivalent to a predicted
One electron reduction of catalyst 1 (Scheme 1) is known to
4
give a ligand-centered reduction of the NNN pincer ligand,
1
production of 0.68 millimoles of H . From quantitative mass
2
as shown by EPR data. In inital voltammetry in MeCN,
a
Department of Chemistry, Yale University, 225 Prospect St.,
New Haven, CT 06520-8107, USA.
E-mail: robert.crabtree@yale.edu, victor.batista@yale.edu,
gary.brudvig@yale.edu; Fax: (+)1 203 432 6144
Department of Geology and Geophysics: Earth Systems Center for
b
Stable Isotopic Studies, Yale University, 210 Whitney Ave.,
New Haven, CT 06511, USA. E-mail: dcolosi@wisc.edu
w Electronic supplementary information (ESI) available: All relevant
experimental details, NMR spectra, kinetics and computational
results. See DOI: 10.1039/c2nj20912h
Scheme 1 Catalyst 1.
This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
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4 4
Fig. 1 Cyclic voltammograms of 2 mM 1 in 0.1 M NBu BF
ꢀ1
ꢀ1
acetonitrile solution: Solid trace: 25 mV s , dashed trace: 50 mV s ,
ꢀ1
dotted trace: 100 mV s
.
spectrometry, 0.65 millimoles of H
2
were detected, also after
background subtraction. Thus, our Faradaic efficiency for H
2
production under these conditions was 95 ꢁ 4%. These data
correspond to a minimum of 65 mole H per mole of Ni per
2
hour (1456 L H produced per mol of Ni catalyst per h). Only
2
a small amount of the soluble catalyst is electroactive at any
one time and thus the observed rate is a lower limit for the
absolute activity.
In order to propose a mechanism of H
2
evolution using
complex 1, we performed ab initio calculations starting with
1
7
the aqua complex of Fig. 2. As in previous studies, we
applied DFT/B3LYP to characterize the structural and spin/
electronic properties of the reaction intermediates and performed
II
Fig. 2 Upper panel: Catalytic cycle based on formation of a Ni -
18
free energy calculations. Gas phase free-energy changes were
calculated at the B3LYP/cc-pVTZ level, using minimum energy
structures obtained at the DFT B3LYP/LANL2DZ level of
theory, and then corrected by solvation free-energy calculations
with the LANL2DZ basis set, using the Polarizable Continuum
hydride intermediate by PCET proposed based on calculations at the
DFT B3LYP/cc-pVTZ level of theory. Lower panel: Reaction free energy
0
+
2
profile showing that PCET enables transformation of [1 (OH )] into
0
+
0
+
0
+
1 (H)] at low voltage. Transformation of [1 (OH )] into [1 (H)] via
[
2
an alternative pathway (shown in grey, lower panel) with sequential
protonation and reduction without invoking PCET is thermodynamically
unfavorable.
19
Model (e = 78.4 for water) as implemented in Gaussian 09.
Consistent with our proposal, the calculations predict that
ꢀ
in aqueous conditions, 1 may readily lose one Br to form a
ꢀ
square planar complex with a single Br ligand (DG for ligand
state. The lowest energy reduction product is the square
ꢀ
1
ꢀ
0
+ 20
planar, water-ligated, complex [1 (OH )] , with high-spin
loss is ꢀ4.7 kcal mol ). Subsequently, the Br ligand can
2
ꢀ
easily exchange with water (DG for replacement of Br with
density on the pincer ligand (as shown by the spin-distribution
in Fig. 3). However, our DFT calculations indicate that an
ꢀ
1
water is ꢀ6.3 kcal mol ). This is consistent with experimental
0
+
isomer of [1 (OH )] with the unpaired electron on the Ni is
results indicating that dissolution of 1 in d -DMSO, an even
6
2
better O-donor ligand than H O, results in the formation of a
2
close in energy to the isomer with the ligand centered radical
and within the errors of DFT we cannot differentiate between
the two. EPR measurements of the one electron reduced
diamagnetic species, most likely a 4-coordinate square planar
dication with coordinated DMSO (see the Fig. S6 in the ESI
for the NMR spectrumw). Given that the catalytic experiments
are performed in water, the DFT results suggest that the
1
3
species provide supporting evidence that the first reduction
process is ligand centered.
0
II
+
)] has been formed, three main pathways are
catalytic cycle begins with a square planar Ni aqua complex
Once [1 (OH
2
I
available: (a) further reduction to give a Ni species with a
and that 1 is only a precatalyst.
0
+
)] to give
III
The first step in the catalytic cycle (Fig. 2) could involve either
II IV
protonation of the Ni starting material to give a formally Ni
ligand-centered radical, (b) protonation of [1 (OH
IV
2
either a Ni hydride with a ligand-centered radical or a Ni
hydride with a neutral ligand, or (c) proton-coupled electron
0
2+
2
hydride or reduction of [1 (OH )] . Both experimental and
II
transfer (PCET) to give a Ni hydride with a neutral ligand.
theoretical data suggest that reduction can be thermodynamically
IV
favored over protonation to the unfavorable Ni oxidation
Our DFT and experimental results suggest that the second
1
150 New J. Chem., 2012, 36, 1149–1152
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has been previously documented: a species related to the
one-electron reduced catalyst precursor 1 has been shown to
1
3
undergo oxidative demethylation upon exposure to air. Our
experiments were performed under rigorous Ar purge and we
believe that such a degradation pathway is only accessible in
the presence of adventitious oxidizing species, such as aerial
dioxygen.z
Conclusions
II
Our Ni pincer complex is an excellent water reduction
Fig. 3 Ligand-centered reduction illustrated by Mulliken spin population
2+
precatalyst in aqueous acid solutions. This seems to be the
first report of such a pincer complex being an operationally
+
0
+
)] (left), trans-[1 (OH
0
analysis of intermediates [1 (OH
2
2 2
) H] (center)
0
2+
and cis-[1 (OH
reduced ligand for [1 (OH
mostly on the Ni center of the Ni-hydride complexes cis-[1 (OH
2 2
) H] (right) showing spin delocalization (in green) in the
0
+
homogeneous catalyst for H reduction. Precatalyst 1 incorporates
2
)] , and localization of the excess spin density
0
2+
a redox-active ligand, a factor that could be important in
2
)
2
H]
0
2+
facilitating the catalytic process, and shows a low overpotential
trans-[1 (OH ) H] . No excess spin is observable in the analyses of
2 2
ꢀ
1
0
2+
0
+
)] , [1 (H)] , and [1 (H
0
2+
)] . Color key population: dark red
[
1 (OH
2
2
for H2 production with a rate of 105 s . Bulk electrolysis
followed by macroscopic determination of the H produced,
demonstrated that complex 1 gives at least 65 mole H per mole
0
2+
+
(
ꢀ0.08)—bright green (+0.27), [1 (OH
2
)] ; dark red (ꢀ0.20)—bright
2
0
green (+0.99), trans-[1 (OH
2+
2
)
2
H] ; dark red (ꢀ0.20)—bright green
2
0
(
+1.04), cis-[1 (OH
2
)
2
H]
.
of catalyst per hour (ꢀ1.1 V vs. NHE in 50 mL 0.1 M KCl/HCl
solution pH 1 with 0.2 mmol catalyst). However, a reliable
I
reduction of 1 to give a Ni species and a ligand-centered
radical is energetically disfavored and requires a significantly
greater reduction potential than that needed during catalysis
2
comparison of H evolution measurements across different
systems is difficult because of the wide array of experimental
11
conditions used in proton reduction. Factors that change
(
vide supra); pathway (a) has therefore been dismissed. Protonation
+
between systems include experimental setups, electrolysis potentials
and choice of solvent and proton source. DFT studies suggest that
the mechanism of proton reduction for 1 involves a key PCET step.
Further mechanistic work is under way to investigate the
proposed intermediates and to tune the properties of the
pincer ligand to enhance the kinetics of proton reduction.
0
of [1 (OH )] could result in the formation of two different Ni
2
0
2+
0
or trans-[1 (OH ) H]
2 2
2+
hydride isomers, cis-[1 (OH ) H]
2
2
that differ in the coordination geometry around Ni. In
2+
1
0
-[1 (OH
2 2
) H] , the hydride is coplanar with the NNN
ligand with two axially-bound water ligands (which stabilize
2+
the higher oxidation state), while in trans-[1 (OH
0
2
)
2
H]
the
hydride is perpendicular to the NNN ligand. The spin density
0
2+
analysis indicates that both cis-[1 (OH
2
)
2
H]
0
2 2
1 (OH ) H] are formally Ni hydrides (see ESI for more
and trans-
Experimental
2+
III
[
IV
Experimental details are given in the supplementary
informationw), suggesting that a Ni species with a ligand-
centered radical is not accessible. However, protonation to
information.w
0
2+
form cis-[1 (OH ) H] or trans-[1 (OH ) H] is significantly
0
2+
2
2
2 2
0
+
0
2+
uphill energetically, (DG ([1 (OH )] /cis-[1 (OH ) H] ) =
aq
2
2 2
Acknowledgements
ꢀ
1
0
+
)] /trans-
4
0
0
kcal mol
2+
(1.73 eV) and DGaq([1 (OH
2
ꢀ1
The work was supported as part of the Center for Electro-
catalysis, Transport Phenomena, and Materials (CETM) for
Innovative Energy Storage, an Energy Frontier Research
Center funded by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences under Award Number
DE-SC00001055 (V. S. B, R. H. C., O. R. L.). We also
gratefully acknowledge the Argonne-Northwestern Solar
Energy Research (ANSER) Center EFRC under Award
Number DE-PS02-08ER15944 (G. W. B, J. D. B.). We thank
Nilay Hazari and his lab for useful discussions as well as
Graham Dobereiner, Kurt Luthy and Dr Jeremy Praetorius
and GE Global Research. We also thank Daryl Smith for help
with the electrochemical cell design.
[
1 (OH
2
)
2
H] ) = 26 kcal mol
(1.13 eV)), and is not
spontaneous. On this basis, we propose that the PCET
+
0
pathway (c) is the most likely route that converts [1 (OH
2
)]
II
0
+
into the square planar Ni hydride [1 (H)] . The free energy
0
+
0
+
requirement for [1 (OH
1.13 + 0.059 pH) eV to protonate [1 (OH
2
)] - [1 (H)] conversion thus includes
0
+
)] to give trans-
(
2
0
2+
) H] minus the excess free energy (4.60 eV) due to
[
1 (OH
2 2
0
2+
the reduction of trans-[1 (OH ) H] to [1 (H)] . Therefore,
0
+
2
2
0
+
by coupling protonation and reduction in PCET, [1 (OH )]
2
+
is converted into [1 (H)] at a potential (1.13 + 0.059
0
2
1
pH ꢀ4.60) V. This very large driving force force of
0
+
0
+
ꢀ1
DGaq([1 (OH
NHE) at pH = 0 is consistent with our measured kinetic
isotope effect (k /k ) of 4.2(1).
The catalytic cycle is completed by protonation of
2
)] /[1 (H)] = ꢀ80 kcal mol (ꢀ0.97 V vs.
H
D
+
II
0
2+
Notes and references
0
[
1 (H)] to give the Ni dihydrogen complex [1 (H
2
)]
)] ) = 9 kcal mol ) and subsequent
evolution, with the H ligand substituted by water to
)
0
+
0
2+
ꢀ1
(
DGaq([1 (H)] /[1 (H
2
z We did not observe evidence of electrodeposition in background
runs performed after running bulk electrolysis and voltammetry.
Relevant backgrounds and controls are available in the ESI.w The
H
2
2
0
2+
regenerate [1 (OH )] . Notably, the highly reactive nature
2
ꢀ
14
1e reduced form of the catalyst was shown to be O sensitive and
2
of the radical intermediates proposed to intervene in the cycle
our measurements were done with rigorous exclusion of air.
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